The drop-down menu shows only courses where the user has the capability
moodle/grade:viewall. The reasoning is that teachers can only fetch grades they would be able to see anyway.
I don't think you can do this easily by adding a new permission to your current Educator role (assuming that is based on the editing teacher archetype role). Teacher-like roles are assigned in the context of individual courses.
What you might be able to is to create a new role named like "All grades viewer" with only this capability
moodle/grade:viewall granted. Then assign this role as a system role (or eventually per course category role) to all your staff who need it.